Panel-shaped formed product and method for producing panel-shaped formed product

ABSTRACT

A panel-shaped formed product made of a metal sheet includes a top panel portion, an opening portion, and a vertical wall portion. The top panel portion is polygonal. The opening portion is provided in the top panel portion. The vertical wall portion extends from at least not less than two adjacent borders among the borders of the top panel portion. At least one pair of each vertical wall portion of pairs of adjacent vertical wall portions of the vertical wall portions has a stepped portion. As a result, the panel shaped formed product has fewer defects such as cracks and wrinkles even when it has a shape that is difficult to form.

TECHNICAL FIELD

The present invention relates to a panel-shaped formed product and amethod for producing a panel-shaped formed product. More specifically,the present invention relates to a panel-shaped formed product suitablefor an automobile door inner panel, and a method for producing the same.

BACKGROUND ART

A door of an automobile is produced by mainly combining a door innerpanel and a door outer panel. The door of an automobile is attached witha window glass, a window driving device, an acoustic speaker, a handle,and the like. To accommodate these, a space is required between the doorinner panel and the door outer panel. For that reason, for example, avertical wall portion is provided on the door inner panel. Also, whenthe door of the automobile is closed, the vehicle interior needs to besealed by the door. For that reason, for example, a step portion isprovided in the vertical wall portion of the door inner panel. As aresult of the step portion of the vertical wall portion facing a pillarof a vehicle body or the like, the tightness of the vehicle interior isensured.

A door inner panel used for a side door of an automobile or the like isformed by subjecting a metal sheet such as a steel sheet to pressworking. In general, since the shape of the door inner panel iscomplicated, the metal sheet may be allowed to significantly deform. Insuch a case, cracks, wrinkles or the like may occur in the formed doorinner panel. Therefore, a metal sheet with high workability is used forthe starting material of the door inner panel. Since the metal sheetwith high workability has low strength, the door inner panel has lowstrength as well. Accordingly, it is often the case that the door innerpanel is attached with a reinforcing member (for example, a beltlinereinforcement, a door impact beam, or the like).

A structure of a door inner panel is disclosed in Japanese PatentApplication Publication No. 2007-296953 (Patent Literature 1), JapanesePatent Application Publication No. 2008-94353 (Patent Literature 2), andJapanese Patent Application Publication No. 2013-112133 (PatentLiterature 3). A method for producing a door inner panel is disclosed inJapanese Patent Application Publication No. 2013-189173 (PatentLiterature 4), Japanese Patent Application Publication No. 2001-38426(Patent Literature 5), Japanese Patent Application Publication No.2011-147970 (Patent Literature 6), and Japanese Patent ApplicationPublication No. 2011-50971 (Patent Literature 7).

A door inner panel disclosed in Patent Literature 1 includes a beltlinereinforcement. The beltline reinforcement is joined along a vehicle bodyfore-and-aft direction in a beltline portion of the door inner panel.Patent Literature 1 describes that such configuration allows thebeltline reinforcement to bear a collision load in the vehicle bodyfore-and-aft direction, thereby effectively reducing the bending momentacting on the beltline portion.

In a door inner panel disclosed in Patent Literature 2, when a collisionload is applied from a side face of the vehicle body, the door innerpanel and a beltline reinforcement are brought into contact with eachother, and a load absorbing portion of the door inner panel is deformed.Patent Literature 2 describes that as a result of such arrangement, theload absorbing portion absorbs part of the load applied in a thicknessdirection of the door inner panel so that the rigidity of the door innerpanel is ensured.

In a side door disclosed in Patent Literature 3, it is configured suchthat a rear end portion and a fore end portion of a beltlinereinforcement formed by hot stamping have a lower strength and a lowerrigidity than those of the main body portion. As a result, when acollision load is applied from the front face of the vehicle body, therear end portion of the beltline reinforcement is plastically deformed,thereby increasing the contact area with a center pillar. PatentLiterature 3 describes that such deformation of the rear end portion ofthe beltline reinforcement can absorb the energy of collision.

In a method for producing a door inner panel disclosed in PatentLiterature 4, a first formed body having a vertical wall portion and asecond formed body are welded together, and the welded formed body isprocessed by hot pressing or rolling. Patent Literature 4 describes thatsuch arrangement makes it possible to provide a longer-length structuralcomponent for a vehicle body, which has excellent load bearingperformance per unit weight thereof.

In a method for producing a door inner panel disclosed in PatentLiterature 5, a wrinkle pressing force during press working of ahigh-strength steel sheet is specified. A relationship between acurvature of wall warping generated during forming and a wrinklepressing force is obtained in advance. On the basis of thisrelationship, a wrinkle pressing force is applied such that thecurvature of wall waring is reduced. Patent Literature 5 describes thatsuch arrangement makes it possible to form a trapezoidal member ofhigh-strength steel sheet with high dimensional accuracy.

In a method for producing a door inner panel disclosed in PatentLiterature 6, a die and a holder of a hot press forming apparatus areoperated together to press-work a sheet to be processed. This makes itpossible to prevent breakage of the sheet to be processed and occurrenceof wrinkles thereof during forming. Further, Patent Literature 6describes that such arrangement can ensure dimensional accuracy of theformed product after forming.

In a method for producing a door inner panel disclosed in PatentLiterature 7, in hot press forming, a spacing between a die and a holderis controlled during processing of a formed product. Patent Literature 7describes that such arrangement makes it possible to suppress wrinklesof a formed product.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.2007-296953

Patent Literature 2: Japanese Patent Application Publication No.2008-94353

Patent Literature 3: Japanese Patent Application Publication No.2013-112133

Patent Literature 4: Japanese Patent Application Publication No.2013-189173

Patent Literature 5: Japanese Patent Application Publication No.2001-38426

Patent Literature 6: Japanese Patent Application Publication No.2011-147970

Patent Literature 7: Japanese Patent Application Publication No.2011-50971

SUMMARY OF INVENTION Technical Problem

However, in the door inner panels of Patent Literatures 1, 2, and 3, aseparate reinforcing member such as a beltline reinforcement isindispensable in order to secure collision characteristics. For thisreason, a door produced from the door inner panels of Patent Literatures1, 2, and 3 has a low manufacturing efficiency and a high cost.Moreover, the steel sheet used for forming the door inner panel is alow-strength soft steel sheet. Therefore, cracks, wrinkles, or the likeare less likely occur in the formed door inner panel. However, when thedoor inner panel has adjacent vertical wall portions, and the verticalwall portion has a step portion, it is likely that cracks, wrinkles orthe like occur.

In the production methods of Patent Literatures 4 to 7, when eachvertical wall portion extending from a border of a top panel portion ofthe formed door inner panel is adjacent to each other, cracks, wrinkles,and the like may occur.

An object of the present invention is to provide a high-strengthpanel-shaped formed product which has fewer defects such as cracks andwrinkles even if it has a shape of a high degree of forming difficulty.Further, another object of the present invention is to provide a methodfor producing a high-strength panel-shaped formed product which hasfewer defects such as cracks and wrinkles even if it is formed into ashape of a high degree of forming difficulty.

Solution to Problem

A panel-shaped formed product made of a metal sheet according to anembodiment of the present invention includes a top panel portion, anopening portion, and a vertical wall portion. The top panel portion ispolygonal. The opening portion is provided in the top panel portion. Thevertical wall portion extends from at least not less than two adjacentborders among borders of the top panel portion. At least one pair ofeach vertical wall portion of pairs of adjacent vertical wall portionsamong the vertical wall portions has a step portion.

A method for producing a panel-shaped formed product according to anembodiment of the present invention is applied to production of apanel-shaped formed product having a top panel portion, a vertical wallportion, and a step portion. The top panel portion is polygonal. Thevertical wall portion extends from at least not less than two adjacentborders among borders of the top panel portion. And, a step portion isprovided in at least one pair of each vertical wall portion of pairs ofadjacent vertical wall portions among the vertical wall portions.

The production method includes a preparation step, a heating step, and apress forming step. In the preparation step, a blank material made of ametal sheet is prepared. In the heating step, the blank material isheated. In the press forming step, the heated blank material issubjected to press working by hot stamping to form the blank materialinto a panel-shaped formed product

The press forming step uses a press working apparatus including a die, afirst punch, a second punch, and a blank holder. The die includes a formengraved portion in which the shape of the panel-shaped formed productis shaped. The first punch has a front end surface which faces the dieand in which the shape of the top panel portion is shaped. The secondpunch is adjacent to the outside of the first punch and has a front endsurface which faces the die and in which the shape of the step portionis shaped. The blank holder is adjacent to at least a portion of theoutside of the second punch, and faces the die. The blank material isdisposed between the die, and the blank holder, the first punch, and thesecond punch. The blank holder, the first punch, and the second punchmove relative to the die, and the first punch and the second punch arepushed in on the blank material to form a panel-shaped formed product.

Advantageous Effects of Invention

A panel-shaped formed product according to the present invention has ahigh strength and fewer defects such as cracks and wrinkles. A methodfor producing a panel-shaped formed product according to the presentinvention enables production of a panel-shaped formed product having ashape of a high degree of forming difficulty while suppressing defectssuch as cracks and wrinkles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a door inner panel according to a firstembodiment.

FIG. 2 is a perspective view of a door inner panel different from thatof FIG. 1.

FIG. 3 is a schematic view of a vertical cross section of a side doorfor an automobile.

FIG. 4 is a schematic view of a horizontal cross section of a side doorfor an automobile and its vicinity.

FIG. 5 is a perspective view of a door inner panel according to a secondembodiment.

FIG. 6 is a perspective view of a door inner panel according to a thirdembodiment.

FIG. 7 is a perspective view of a door inner panel according to a fourthembodiment.

FIG. 8 is a schematic view of a hot stamping apparatus used for pressforming of the door inner panel of the present embodiment.

FIG. 9A shows a stage of clamping a blank material with a blank holderin a press forming step of the present embodiment.

FIG. 9B shows a state in which pushing in by a second punch is completedin the press forming step of the present embodiment.

FIG. 9C shows a state in which pushing in by a first punch is completedin the press forming step of the present embodiment.

FIG. 10 is a cross-sectional view showing a state during press workingby a general hot stamping apparatus.

FIG. 11A shows a stage of clamping a blank material with a blank holderin a press forming step different from that of FIGS. 9A to 9C.

FIG. 11B shows a state in which pushing in by the second punch iscompleted in a press forming step different from that of FIGS. 9A to 9C.

FIG. 11C shows a state in which pushing in by the first punch iscompleted in a press forming step different from that of FIGS. 9A to 9C.

FIG. 12 is a perspective view of a door inner panel of a fifthembodiment.

FIG. 13A is a perspective view of a starting material of the door innerpanel shown in FIG. 12, showing a state before being punched an openingportion.

FIG. 13B is a perspective view of a starting material of the door innerpanel shown in FIG. 12, showing a state after being punched an openingportion and immediately before being subjected to hot stamping.

FIG. 14 is a perspective view of a door inner panel of a sixthembodiment.

FIG. 15 is a perspective view of the door inner panel of a seventhembodiment.

FIG. 16A is a perspective view of a starting material of the door innerpanel shown in FIG. 15, showing a state before being punched an openingportion.

FIG. 16B is a perspective view of a starting material of the door innerpanel shown in FIG. 15, showing a state after being punched an openingportion and immediately before being subjected to hot stamping.

FIG. 17 is a perspective view of a door inner panel of an eighthembodiment.

FIG. 18 is a perspective view of a door inner panel of a ninthembodiment.

FIG. 19A is a perspective view of the starting material of the doorinner panel shown in FIG. 18, showing a state before being punched anopening portion.

FIG. 19B is a perspective view of the starting material of the doorinner panel shown in FIG. 18, showing a state after being punched anopening portion and immediately before being subjected to hot stamping.

FIG. 20 is a perspective view of a door inner panel different from thatof FIG. 18.

FIG. 21 is a perspective view of the starting material of the door innerpanel shown in FIG. 20, showing a state after being punched an openingportion and immediately before being subjected to hot stamping.

FIG. 22 is a perspective view of a door inner panel of a tenthembodiment.

FIG. 23A is a perspective view of a starting material of the door innerpanel shown in FIG. 22, showing a state before being punched an openingportion.

FIG. 23B is a perspective view of the starting material of the doorinner panel shown in FIG. 22, showing a state after being punched anopening portion and immediately before being subjected to hot stamping.

FIG. 24 shows dimensions of a die used in the analysis of the presentexample.

FIG. 25A shows a sheet thickness reduction rate of a door inner panel ofInventive Example 1.

FIG. 25B shows curvature distribution of the door inner panel ofInventive Example 1.

FIG. 26A shows a sheet thickness reduction rate of a door inner panel ofInventive Example 2.

FIG. 26B shows curvature distribution of the door inner panel ofInventive Example 2.

FIG. 27A shows a sheet thickness reduction rate of a door inner panel ofComparative Example.

FIG. 27B shows curvature distribution of the door inner panel ofComparative Example.

DESCRIPTION OF EMBODIMENTS

A panel-shaped formed product made of a metal sheet according to anembodiment of the present invention includes a top panel portion, anopening portion, and a vertical wall portion. The top panel portion ispolygonal. The opening portion is provided in the top panel portion. Thevertical wall portion extends from at least not less than two adjacentborders among borders of the top panel portion. At least one pair ofeach vertical wall portion of pairs of adjacent vertical wall portionsamong the vertical wall portions has a step portion.

The panel-shaped formed product of the present embodiment includesvertical wall portions which are adjacent to each other and have a stepportion. As a result, the panel-shaped formed product of the presentembodiment can enhance the tightness of vehicle interior.

Preferably, the panel-shaped formed product has a tensile strength ofnot less than 1200 MPa.

Applying such a panel-shaped formed product to a door inner panel for anautomobile will result in improvement of collision characteristicthereof.

Preferably, the panel-shaped formed product is a door inner panel for anautomobile, which has no vertical wall portion on a border on thevehicle upper side among borders of the top panel portion. Thus, thedoor inner panel can be combined with the door outer panel to form aside door for an automobile. A side window glass and others areaccommodated between the door outer panel and the door inner panel

Preferably, the opening portion is provided so as to leave a peripheraledge portion of the top panel portion. As a result, an edge portion onthe vehicle upper side including the border on the vehicle upper sideout of the peripheral edge portion of the top panel portion forms abeltline portion of the door inner panel.

Preferably, an edge portion on the vehicle upper side including a borderon the vehicle upper side of the top panel portion is provided with atleast one of a recessed portion and a protruded portion along the edgeportion on the vehicle upper side. In this case, the upper edge on thevehicle upper side of the top panel portion (beltline portion) isresponsible for the role of a beltline reinforcement that reinforces thetop panel portion. In other words, the door inner panel and the beltlinereinforcement are integrated. As a result, it is possible to reduce theweight and the production cost of the door inner panel.

Preferably, the top panel portion of the panel-shaped formed product hasa boundary portion for dividing the opening portion into a plurality ofportions. In the boundary portion, at least one of a recessed portionand a protruded portion is provided along the boundary portion. In thiscase, the boundary portion of the top panel portion plays a role of adoor impact beam that reinforces the top panel portion. In other words,the door inner panel and the door impact beam are integrated. As aresult, it is possible to reduce the weight and the production cost ofthe door inner panel.

Preferably, the metal sheet is a steel sheet. In this case, thepanel-shaped formed product, which can be formed by hot stamping, has ahigh strength and fewer defects such as cracks and wrinkles.

In the case of a door inner panel, the sheet thickness of a region of alower-side vertical wall portion including a border on the lower side ofthe top panel portion is larger than the sheet thickness of a regionadjacent to that region. The lower-side vertical wall portion having alarge sheet thickness will improve collision characteristic.

In the case of the door inner panel described above, preferably, thesheet thickness of a region of a front-side vertical wall portionincluding a border on the front side of the top panel portion is largerthan the sheet thickness of a region adjacent to that region. Thefront-side vertical wall portion corresponds to an attaching portion ofa hinge to be connected to the pillar. The front-side vertical wallportion having a large sheet thickness sufficiently ensures the strengthof the hinge attaching portion.

In the case of the door inner panel described above, preferably, thesheet thickness of a region of an edge portion on the vehicle upper sideincluding a border on the vehicle upper side of the top panel portion islarger than the sheet thickness of a region adjacent to that region. Theedge portion on the vehicle upper side including the border on thevehicle upper side out of the peripheral edge portion of the top panelportion forms a beltline portion of the door inner panel. The edgeportion on the vehicle upper side having a large sheet thicknessimproves collision characteristics.

In the case of the door inner panel described above, preferably, the toppanel portion has a boundary portion which divides the opening portioninto a plurality of portions, and the sheet thickness of a region of theboundary portion is larger than the sheet thickness of a region adjacentto that region. The boundary portion having a large sheet thicknessimproves collision characteristics.

A reinforcing member such as a beltline reinforcement or a door impactbeam may be added to the door inner panel described above. When thesereinforcing members are added, the reinforcing members may be attachedso as to be superposed on the above-described recessed portion or convexportion, etc. or may be attached to another area. In that case, evenwhen a material having a lower cost than that of a conventionalreinforcing material, such as a thin sheet material and a low-strengthmaterial is used, it is possible to satisfy various characteristics.Further, it is also possible to simplify the shape of the reinforcingmember to be attached additionally. For that reason, even when thereinforcing member is added, it is possible to suppress the productioncost.

A method for producing a panel-shaped formed product according to anembodiment of the present invention is applied to production of apanel-shaped formed product including a top panel portion, a verticalwall portion, and a step portion. The top panel portion is polygonal.The vertical wall portion extends from at least not less than twoadjacent borders among borders of the top panel portion. And a stepportion is provided in at least one pair of each vertical wall portionof pairs of adjacent vertical wall portions among the vertical wallportions.

The production method includes a preparation step, a heating step, and apress forming step. In the preparation step, a blank material made of ametal sheet is prepared. In the heating step, the blank material isheated. In the press forming step, the heated blank material issubjected to press working by hot stamping, thereby forming the blankmaterial into a panel-shaped formed product.

In the press forming step, a press working apparatus including a die, afirst punch, a second punch, and a blank holder is used. The die has adie engraved portion in which the shape of the panel-shaped formedproduct is shaped. The first punch has a front end surface which facesthe die and in which the shape of the top panel portion is shaped. Thesecond punch is adjacent to the outside of the first punch and has afront end surface which faces the die and in which the shape of the stepportion is shaped. The blank holder is located adjacent to at least aportion of the outside of the second punch and faces the die. The blankmaterial is disposed between the die, and the blank holder, the firstpunch, and the second punch. The blank holder, the first punch, and thesecond punch move relative to the die, and the first punch and thesecond punch are pushed in on the blank material to form a panel-shapedformed product.

The method for producing a panel-shaped formed product of the presentembodiment enables production of a panel-shaped formed product having ashape of a high degree of forming difficulty, while suppressing defectssuch as cracks and wrinkles. The shape of a high degree of formingdifficulty includes, for example, one in which adjacent vertical wallportions of the panel-shaped formed product have a step portion.

Preferably, the blank material has an opening portion at a positioncorresponding to a top panel portion of the panel-shaped formed product.

As a result of that, the top panel portion is formed through stretchflange forming. Thus, it is possible to suppress cracks, wrinkles, etc.of the panel-shaped formed product.

Preferably, in the press forming step, pushing in on the blank materialby the second punch is completed prior to pushing in on the blankmaterial by the first punch.

In this case, in the press forming step, the pushing in on the blankmaterial by the first punch may be started when the pushing in on theblank material by the second punch is completed or after the pushing inis completed. Moreover, in the press forming step, the pushing in on theblank material by the first punch may be started before the pushing inon the blank material by the second punch is completed.

As a result, the second punch together with the die holds down the blankmaterial prior to the first punch. As a result, it is possible tofurther suppress cracks, wrinkles, etc. of the panel-shaped formedproduct. Holding down the blank material means that the blank materialis completely clamped by the punch and the die so as not to be pushed infurther. Further, completion of pushing in on the blank material by eachindividual punch means that the blank material becomes held down.

Preferably, the tensile strength of the panel-shaped formed productafter forming is not less than 1200 MPa.

Applying such a panel-shaped formed product to a door inner panel for anautomobile allows improvement in collision characteristics.

Preferably, the die engraved portion of the die used in the pressforming step satisfies the following conditions: d2≥40 mm, andd1/d2<0.8, where d1 is a depth from a reference surface facing the blankholder to a step surface facing the second punch, and d2 is a depth fromthe reference surface to a die bottom surface facing the first punch.

In this case, applying the panel-shaped formed product to a door innerpanel for an automobile allows to obtain a sufficient amount of spacefor accommodating the window glass and others. It is also possible toimprove the tightness of vehicle interior.

The metal sheet described above is preferably a steel sheet. In thiscase, the strength of the formed panel-shaped formed product isincreased by quenching through hot stamping. The steel sheet may be atailored blank. This makes it possible to enhance strength only in anecessary place, and also to reduce the sheet thickness.

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. The same or corresponding partsare denoted by the same reference symbols, and description thereof willnot be repeated. In the present embodiments, a case of a door innerpanel for a vehicle, in which a panel-shaped formed product is made ofsteel sheet, will be described as an example.

First Embodiment

FIG. 1 is a perspective view of a door inner panel of a firstembodiment. Referring to FIG. 1, a door inner panel 1 includes: a toppanel portion 2; an opening portion 3; a vertical wall portion 4, a stepportion 5, and a flange portion 6. The planar shape of the top panelportion 2 is polygonal. The polygon may be, for example, a quadrangle ora pentagon. A corner part of a polygon may have a round shape. FIG. 1shows an exemplary case in which the planar shape of the top panelportion 2 is a pentagon. In the door inner panel 1, a border on thevehicle upper side of the top panel portion 2 forms a beltline BL. Inthe present embodiment, a case in which the sheet thickness of the steelsheet is constant will be described. Therefore, the sheet thickness ofthe door inner panel 1 is also constant over the entire area. In astrict sense, however, a slight increase or decrease of the sheetthickness will be caused by press forming.

The vertical wall portion 4 extends from at least not less than twoadjacent borders among borders of the top panel portion 2. FIG. 1 showsan exemplary case in which the vertical wall portion 4 extends from fourborders 2A, 2B, 2C, and 2D excluding the border (beltline BL) on thevehicle upper side among five borders of the pentagonal top panelportion 2. However, the number of borders of the top panel portion 2from which the vertical wall portion 4 extends is not limited to four.The vertical wall portion 4 may extend from each of two or more adjacentborders of the top panel portion 2. When the vertical wall portion 4extends from each border of the two or more adjacent borders of the toppanel portion 2, each vertical wall portion extending from each borderwill be adjacent to each other. FIG. 1 shows an exemplary case in whichthe vertical wall portion 4 extends perpendicular to the top panelportion 2. However, the vertical wall portion 4 may not be strictlyperpendicular to the top panel portion 2. The top panel portion 2 isprotruded from the flange portion 6 by the vertical wall portion 4,thereby forming a space for accommodating a window glass and others.

The step portion 5 extends outward from a vertical wall portion 4Aconnected to the top panel portion 2. An outer edge of the step portion5 is connected to a vertical wall portion 4B connected to the flangeportion 6. FIG. 1 shows an exemplary case in which the surface of thestep portion 5 is parallel with the top panel portion 2. However, thesurface of the step portion 5 may not be strictly parallel with the toppanel portion 2. FIG. 1 shows an exemplary case in which three adjacentvertical wall portions 4 have a step portion 5. In other words, a casein which there are two pairs of adjacent vertical wall portions 4, andthe two pairs each have the step portion 5. However, the number of pairsof the vertical wall portions 4 having the step portion 5 is not limitedto two. At least one pair among pairs of adjacent vertical wall portions4 may have a step portion 5. FIG. 1 shows an exemplary case in which onestep of step portion 5 is provided in the vertical wall portion 4.However, the number of steps of the step portion 5 is not limited toone, but may be plural.

The top panel portion 2 includes an opening portion 3. When the doorinner panel 1 is made of a steel sheet, the top panel portion 2 isformed through stretch flange deformation. Since the top panel portion 2has the opening portion 3, it is susceptible to stretch flangedeformation. FIG. 1 shows an exemplary case in which an opening portion3 is provided at one location so as to leave a peripheral edge portionof the top panel portion 2. However, the number of locations of theopening portion 3 is not limited to one. The top panel portion 2 mayinclude a plurality of opening portions 3. The shape of the openingportion 3 may be circular, elliptic, polygonal, or the like, and is notparticularly limited. In the door inner panel 1 for an automobile, anacoustic speaker, a handle, and the like are attached in the openingportion 3.

FIG. 2 is a perspective view of a door inner panel having an openingportion 3 having a shape different from that in FIG. 1. Referring toFIG. 2, the opening portion 3 extends over one border of a peripheraledge portion of a top panel portion 2. In other words, the openingportion 3 is provided such that a beltline BL is interrupted. In thiscase, a reinforcing member such as a beltline reinforcement may beattached to the beltline BL.

FIG. 3 is a schematic view of a vertical cross section of a side doorfor an automobile. Referring to FIG. 3, the side door is produced bycombining a door outer panel A and a door inner panel 1. A space SP is aspace between the door outer panel A and the door inner panel 1. Anacoustic speaker, a window glass, a window drive device, and the likemay be accommodated in the space SP. For that reason, it is often thecase that the space SP is formed by providing a vertical wall portion 4in the door inner panel 1.

In the door inner panel 1, a border on a vehicle upper side of the toppanel portion 2 is a beltline BL. The beltline BL is located on anentrance/exit side of a window (not shown). When the window is opened,the window glass is lowered and accommodated in the space SP. Therefore,there is no vertical wall portion in the beltline BL.

FIG. 4 is a schematic view of a horizontal cross section of a side doorfor an automobile and its vicinity. Referring to FIG. 4, the stepportion 5 faces a pillar B of the vehicle body. For that reason, thedoor inner panel 1 of the present embodiment allows to improve thetightness of vehicle interior of an automobile compared to a door innerpanel having no step portion 5. A seal member may be disposed betweenthe step portion 5 and the pillar B. In this case, it is possible tofurther improve the tightness of vehicle interior. The seal member is,for example, a rubber.

A hinge to be connected to a pillar (not shown) is attached to thevertical wall portion 4 on the vehicle front side including a border 2Aof the top panel portion 2 on the vehicle front side.

The symbol h1 shown in FIG. 4 indicates a height from the flange portion6 to the step portion 5. And h2 indicates a height from the flangeportion 6 to the top panel portion 2. The door inner panel 1 of thepresent embodiment preferably satisfies conditions: h2≥40 mm, andh1/h2<0.8. This is because when h2<40 mm, the space SP that accommodatesa window glass and others is too small. Also that is because whenh1/h2≥0.8, the tightness of vehicle interior deteriorates as a result ofa small distance between the top panel portion 2 and the step portion 5.

Second Embodiment

FIG. 5 is a perspective view of a door inner panel of a secondembodiment. A door inner panel 1 of a second embodiment shown in FIG. 5is based on the door inner panel 1 of the first embodiment describedabove. The same applies to a third and fourth embodiments to bedescribed later, and overlapping explanation will be appropriatelyomitted.

Referring to FIG. 5, in the door inner panel 1 of the second embodiment,a step portion 5 is provided in all of the four adjacent vertical wallportions 4 (three pairs of vertical wall portions 4). In this case, thestep portion 5 faces a center pillar, a front pillar, a side sill, andthe like of the vehicle body. Accordingly, it is possible to furtherimprove the tightness of vehicle interior.

Third Embodiment

FIG. 6 is a perspective view of a door inner panel of a thirdembodiment. Referring to FIG. 6, a door inner panel 1 of the thirdembodiment shows a case in which a vertical wall portion 4 extends fromfour borders except the beltline BL out of the five borders of the toppanel portion 2 having a pentagon shape, and a step portion 5 isprovided in three vertical wall portions 4 of four adjacent verticalwall portions 4.

In the door inner panel 1 of the third embodiment, there is provided inan edge portion 21 on the vehicle upper side including a beltline BL ofthe top panel portion 2 (hereinafter referred to as a “beltlineportion”), a recessed portion 7 along that beltline portion 21. Formingthe recessed portion 7 increases a cross-sectional secondary moment ofthe beltline portion 21 of the top panel portion 2. In other words, therecessed portion 7 increases the strength of the beltline portion 21. Ingeneral, the beltline portion of a door inner panel is often reinforcedby attaching a reinforcing member such as a beltline reinforcementthereto. In this regard, in the door inner panel 1 of the thirdembodiment, a beltline portion 21 having a recessed portion 7 isprovided integrally in the top panel portion 2. In other words, in thethird embodiment, the beltline reinforcement is integrated into the doorinner panel. Therefore, the beltline portion 21 does not need to bereinforced by a separate beltline reinforcement. As a result of that, itis possible to reduce the number of components and assembly man-hours ofa door, and further to reduce the weight of the door.

A protruded portion in place of the recessed portion 7 may be providedin the beltline portion 21, or both of the recessed portion 7 and theprotruded portion may be provided. Forming the protruded portion willequally increase the cross-sectional secondary moment of the beltlineportion 21 of the top panel portion 2.

Fourth Embodiment

FIG. 7 is a perspective view of a door inner panel of a fourthembodiment. Referring to FIG. 7, in a door inner panel 1 of the fourthembodiment, the opening portion of the first embodiment shown in FIG. 1is divided into multiple portions. In other words, a plurality ofopening portions are provided in the top panel portion 2. FIG. 7 showsan exemplary case in which two opening portions 3A and 3B are provided.

In the door inner panel 1 of the fourth embodiment, the top panelportion 2 includes a boundary portion 22 between the opening portions 3Aand 3B. The boundary portion 22 is provided with a recessed portion 8along the boundary portion 22. Forming the recessed portion 8 increasesthe cross-sectional secondary moment of the boundary portion 22 of thetop panel portion 2. In other words, the recessed portion 8 increasesthe strength of the boundary portion 22. Generally, in order to enhancethe strength of a side door, the top panel portion of the door innerpanel is often reinforced by attaching a reinforcing member such as adoor impact beam thereto. In this regard, in the door inner panel 1 ofthe fourth embodiment, a boundary portion 22 having a recessed portion 8is integrally provided in the top panel portion 2. In other words, inthe fourth embodiment, a door impact beam is integrated with the doorinner panel. Accordingly, the top panel portion 2 does not need to bereinforced by a door impact beam or the like. As a result, it ispossible to reduce the number of components and assembly man-hours of adoor, and further to reduce the weight of the door.

A protruded portion in place of the recessed portion 8 may be providedin the boundary portion 22, or both of the recessed portion 8 and theprotruded portion may be provided. Forming the protruded portion willequally increase the cross-sectional secondary moment of the boundaryportion 22 of the top panel portion 2.

Either one or both of the recessed portion 7 and the protruded portionshown in FIG. 6 may be added to the beltline portion 21 of the doorinner panel 1 of the fourth embodiment shown in FIG. 7.

Here, a panel-shaped inner panel like a door inner panel, in which astep portion is provided at vertical wall portions adjacent to eachother, is very difficult to be formed, and is likely to have defectssuch as cracks and wrinkles at the time of press forming. For thatreason, conventionally, a low-strength steel sheet having high ductilityis used as the starting material when forming a formed product having acomplicated shape. As a result of that, there has been a limit in theimprovement of collision characteristics of a panel-shaped formedproduct. In this regard, by providing an opening portion in the toppanel portion and applying hot stamping thereto, defects such as cracks,wrinkles, and the like are suppressed even when the formed product has acomplicated shape having a vertical wall portion and a step portion,making it possible to obtain a high-strength panel-shaped formed producthaving a tensile strength of not less than 1200 MPa.

The steel sheet to be used as the starting material of the panel-shapedformed product of the present embodiment preferably contains, in mass %,carbon (C): not less than 0.11%. When the steel sheet contains not lessthan 0.11% of carbon, it is possible to increase the strength of thepanel-shaped formed product after hot stamping.

By increasing the strength of the panel-shaped formed product asdescribed above, and applying it to a door inner panel of an automobile,the collision characteristics of a door will be improved. The hardnessof the panel-shaped formed product is preferably not less than HV 380 inthe Vickers hardness. A hardness of HV 380 corresponds to a tensilestrength of 1200 MPa. The Vickers hardness HV conforms to JIS Z 2244.

A door inner panel having a high tensile strength is usually difficultto be formed. Hereinafter, an example of a method for producing the doorinner panel of the above-described embodiments will be described. Thefollowing production method shows an exemplary case in which the doorinner panel to be produced is made of a steel sheet and has a tensilestrength of not less than 1200 MPa.

[Production Method]

A method for producing a door inner panel of the present embodimentincludes a preparation step, a heating step, and a press forming step byhot stamping. In the preparation step, a blank material made of a steelsheet is prepared. In the heating step, the blank material is heated. Inthe press forming step, the heated blank material is subjected to pressworking, and at the same time, the formed door inner panel is quenched.In the press forming step of the present embodiment, a hot stampingapparatus is used as a press working apparatus.

[Hot Stamping Apparatus 10]

FIG. 8 is cross sectional view to schematically show a hot stampingapparatus for producing a door inner panel of the present embodiment.Referring to FIG. 8, the hot stamping apparatus 10 includes a punch 11and a blank holder 14 as an upper die, and a die 15 as a lower die.

The punch 11 includes a first punch 12 and a second punch 13. The firstpunch 12 includes a front end surface 12A. The front end surface 12A ofthe first punch 12 is shaped with a shape of a top panel portion of thedoor inner panel. The second punch 13 includes a front end surface 13A.The front end surface 13A of the second punch 13 is shaped with theshape of the step portion of the door inner panel. The punch 11 pushesin a blank material S on a die engraved portion 16 of the die 15 to forma door inner panel.

The blank holder 14 is disposed adjacent to at least a part of theoutside of the second punch 13. The blank holder 14 has a front endsurface 14A. The front end surface 14A of the blank holder 14 faces areference surface 16C of the die 15. The blank holder 14 clamps theblank material S between itself and the reference surface 16C of the die15.

The die 15 includes a die engraved portion 16. The die engraved portion16 includes a die bottom surface 16A, a step surface 16B, and areference surface 16C. The die bottom surface 16A faces the front endsurface 12A of the first punch 12. The step surface 16B faces the frontend surface 13A of the second punch 13.

The first punch 12, the second punch 13, and the blank holder 14 aresupported by an upper die holder 17. A pressurizing member (not shown)is provided between the second punch 13 and the blank holder 14, and theupper die holder 17. The pressurizing member is a hydraulic cylinder, agas cylinder, a spring, a rubber, or the like. The die 15 is secured toa lower die holder 18. The upper die holder 17 is attached to a slide(not shown). The lower die holder 18 is attached to a bolster plate (notshown). Here, the hot stamping apparatus 10 is not limited to the caseshown in FIG. 8. For example, the first punch 12, the second punch 13,and the blank holder 14 may be individually attached to a movable slide.

The die bottom surface 16A and the front end surface 12A of the firstpunch 12 forms the top panel portion 2 of the door inner panel 1 shownin FIG. 1. The step surface 16B and the front end surface 13A of thesecond punch 13 form the step portion 5 of the door inner panel 1 shownin FIG. 1. The reference surface 16C and the front end surface 14A ofthe blank holder 14 form the flange portion 6 of the door inner panel 1shown in FIG. 1.

In the die 15, a depth d1 from the reference surface 16C to the stepsurface 16B, and a depth d2 from the reference surface 16C to the diebottom surface 16A preferably satisfy conditions: d2≥40 mm, andd1/d2<0.8. The depth d1 and the depth d2 respectively correspond to theheight h1 and the height h2 shown in FIG. 4. Therefore, when d2<40 mm,the space for accommodating a window glass and others is too small.Moreover, when d1/d2≥0.8, since the distance between the top panelportion 2 and the step portion 5 is small, the tightness of vehicleinterior deteriorates.

The hot stamping apparatus 10 of the present embodiment shows a case inwhich it includes a punch 11 and a blank holder 14 above the apparatus,and the die 15 below the apparatus. However, the arrangement will not belimited to the case shown in FIG. 8. In other words, in the hot stampingapparatus 10, the arrangement of the punch 11 and the blank holder 14,and the die 15 may be inverted upside down. In short, any configurationmay be adopted provided that the punch 11 and the blank holder 14 arerelatively movable with respect to the die 15. Hereinafter, the steps ofthe production method of the present embodiment will be described.

[Preparation Step]

In the preparation step, a blank material made of a steel sheet isprepared. The steel sheet of the door inner panel of the presentembodiment preferably contains carbon (c): not less than 0.11%. When thesteel sheet contains not less than 0.11% of carbon, it is possible toincrease the strength of the door inner panel after hot stamping.

[Heating Step]

In the heating step, the blank material is heated by a heating apparatus(not shown). When the blank material is a steel sheet, the heatingtemperature is preferably not less than 700° C. The heating temperatureis, for example, 900° C. The heating temperature is appropriately setdepending on a material type, a degree of forming difficulty, or thelike. In the hot stamping, since the blank material is heated to besoftened, it is possible to form a complicated shape. The complicatedshape includes, for example, a shape like a door inner panel 1 shown inFIG. 1, in which adjacent vertical wall portions 4 have a step portion5.

Preferably, the blank material is heated to not less than an A1transformation point of the material. The blank material is morepreferably heated to not less than an A3 transformation point. In thehot stamping, concurrently with the blank material being press formed,the formed door inner panel is quenched. When the blank material isheated to not less than the A1 point, the door inner panel afterquenching will have a martensitic structure, and thus a high strength.

[Press Forming Step]

FIGS. 9A to 9C are cross sectional views schematically showing the pressforming step of the present embodiment. FIG. 9A shows a stage ofclamping a blank material S with a blank holder 14. FIG. 9B shows astate when pushing in by the second punch 13 is completed. FIG. 9C showsa state when pushing in by the first punch 12 is completed.

Referring to FIG. 9A, the heated blank material S is placed in the hotstamping apparatus 10. After the blank material S is placed, the slidemoves down. This causes the blank material S to be clamped between thefront end surface 14A of the blank holder 14 and the reference surface16C of the die 15. However, the spacing between the front end surface14A of the blank holder 14 and the reference surface 16C of the die 15is preferably larger than the thickness of the blank material S. Inother words, a gap is provided between the blank material S and thefront end surface 14A of the blank holder 14. The size of the gap is,for example, 0.1 mm. When the blank material S is brought into contactwith the blank holder 14, a portion of the blank material S, which isbrought into contact with the blank holder 14, is cooled before theblank material S is press formed. For that reason, the cooling rate ofthe blank material S varies at locations, and therefore the strength ofthe formed product varies at locations. Therefore, it is preferable thata slight gap is provided between the front end surface 14A of the blankholder 14 and the blank material S.

Referring to FIG. 9B, as the slide further moves down, the blankmaterial S is pointing formed by the punch 11 and the die 15. FIG. 9Bshows a case in which when pushing in on the blank material S by thesecond punch 13 is completed, the front end surface 12A of the firstpunch 12 is located at the same height as the front end surface 13A ofthe second punch 13. In other words, it shows a case in whichconcurrently with completion of pushing in on the blank material S bythe second punch 13, pushing in on the blank material S by the firstpunch 12 is started. However, when pushing in on the blank material S bythe second punch 13 is completed, the height of the front end surface12A of the first punch 12 is not limited to the same height as that ofthe front end surface 13A of the second punch 13. The front end surface12A of the first punch 12 at this moment may be located either at ahigher position or at a lower position than that of the front endsurface 13A of the second punch 13. In other words, after the pushing inon the blank material S by the second punch 13 is completed, the pushingin on the blank material S by the first punch 12 may be started.Moreover, before the pushing in on the blank material S by the secondpunch 13 is completed, the pushing in on the blank material S by thefirst punch 12 may be started. In either case, the pushing in by thefirst punch 12 will not be completed prior to the pushing in by thesecond punch 13. Note that holding down of the blank material by theblank holder and the die only needs to be performed by the time theforming by the second punch is completed.

Referring to FIG. 9C, after the pushing in by the second punch 13 iscompleted, the first punch 12 moves down and the blank material S ispointing formed. At this moment, a portion of the blank material S,which has been pushed in by the second punch 13, is restricted by thesecond punch 13. This allows suppression of wrinkles which occur in thevicinity of the step portion of the door inner panel. This point will bedescribed in detail below.

[Suppression of Cracks and Wrinkles]

FIG. 10 is a cross-sectional view showing a state during press workingby a common hot stamping apparatus. FIG. 10 shows an enlarged view ofthe vicinity of a step surface of the die of the common hot stampingapparatus. Referring to FIG. 10, in a hot stamping apparatus 200, frontend surfaces 210A and 210B of a punch 210 are integrally shaped with thepunch 210. As a result of that the front end surfaces 210A and 210B ofthe punch 210 simultaneously reach a die bottom surface 220A and a stepsurface 220B of a die 220. The distance over which the front end surface210A pushes in on the blank material S is longer than that of the frontend surface 210B. As a result of that, as shown in FIG. 10, when thepunch 210 is moved down, first, the front end surface 210A pushes in onthe blank material S. At this moment, a portion S1 of the blank materialS is not restricted by the front end surface 210B of the punch 210 andthe step surface 220B of the die 220. In other words, the portion S1 ofthe blank material S will not come into contact with the front endsurface 210B of the punch 210 and the step surface 220B of the die 220.

In hot stamping, the blank material is cooled by the contact between theblank material, and the punch, the die, and the like. Therefore, in thestage shown in FIG. 10 during press working, a portion S1 of the blankmaterial S is not cooled. The portion S1 of the blank material S iscooled when the punch 210 is further pushed in from the position shownin FIG. 10. In short, when a door inner panel in which a step portion isprovided in a vertical wall portion is formed by a punch 210 in whichshapes of the top panel portion and the step portion of the door innerpanel are integrally shaped, a portion S1 of the blank material S iscooled slower than the other portions.

If the cooling of the blank material S is partially delayed, thestrength and ductility of the blank material S may be partiallydifferent. In this case, cracks and wrinkles are more likely to occur inthe formed door inner panel. As shown in FIG. 1, when adjacent verticalwall portions 4 of the door inner panel 1 have a step portion 5,particularly cracks and wrinkles become more likely to occur. When thestrength of the door inner panel after forming is high, cracks, wrinklesand the like become further likely to occur.

A method for producing a door inner panel of the present embodimentuses, as shown in FIG. 8, a hot stamping apparatus 10 including a firstpunch 12 and a second punch 13. As a result, the top panel portion 2 andthe step portion 5 of the door inner panel 1 as shown in FIG. 1 areformed by separate punches. In addition, pushing in by the second punch13 is completed prior to pushing in by the first punch 12. Thereby, whenone of the punches forms the top panel portion 2, the other punch holdsdown the step portion 5 of the door inner panel 1. Therefore, when thetop panel portion 2 is formed, an unrestricted portion of the blankmaterial is reduced, thereby allowing to suppress cracks and wrinkles ofthe door inner panel.

The blank material S may have an opening portion. In this case, theblank material S has an opening portion at a position facing the diebottom surface 16A of the die 15. As a result, as shown in FIG. 1, adoor inner panel 1 having an opening portion 3 in the top panel portion2 is formed. In other words, the opening portion of the blank material Scorresponds to the opening portion 3 of the door inner panel 1. When theblank material S has an opening portion, the top panel portion 2 isformed by stretch flange forming. Specifically, when the first punch 12processes the blank material S, an outer edge of the opening portionextends in a direction in which the opening portion extends. Therefore,cracks are less likely to occur even when the first punch 12 is pushedin. Further, in hot stamping, since the blank material S is heated, itsductility is improved, thus facilitating stretch flange forming.

[Other Production Methods]

FIGS. 11A to 11C are cross-sectional views to schematically show a pressforming step, which is different from that of FIGS. 9A to 9C. FIG. 11Ashows a stage of clamping a blank material S with a blank holder 14.FIG. 11B shows a state when pushing in by the second punch 13 iscompleted. FIG. 11C shows a state when pushing in by the first punch 12is completed

Referring to FIG. 11B, in a press forming step which is different fromthat in FIG. 9A to 9C, when pushing in on the blank material S by thesecond punch 13 is completed, the front end surface 12A of the firstpunch 12 is located below the front end surface 13A of the second punch13. In other words, before pushing in on the blank material S by thesecond punch 13 is completed, the pushing in on the blank material S bythe first punch 12 is started. At this time, the pushing in on the blankmaterial S by the first punch 12 has not been completed. In this case aswell, the second punch 13 holds down the blank material S before thepushing in by the first punch 12 is completed. As a result, even if adoor inner panel having a shape of a high degree of forming difficultyis formed, it is possible to suppress cracks, wrinkles, and the like.FIGS. 11A to 11C show a case in which a blank material S has an openingportion 31. Accordingly, the top panel portion of the door inner panelis formed by stretch flange forming.

In hot stamping, quenching is performed simultaneously with the formingof the blank material. Specifically, the blank material is cooled bycontact with a punch, a die, and a holder. As a result, a high-strengthdoor inner panel can be formed. The high-strength door inner panel is,for example, a formed product having a tensile strength of not less than1200 MPa.

In a production method of the present embodiment, the height position ofthe front end surface 12A of the first punch 12 is not particularlylimited before pushing in by the second punch 13 is completed. In short,the first punch 12 may first push in the blank material S, or the secondpunch 13 may first push in the blank material S. It is only needed thatthe pushing in on the blank material S by the second punch 13 iscompleted prior to the pushing in on the blank material S by the firstpunch 12. As a result, it is possible to form a door inner panel havinga shape of a high degree of forming difficulty. Moreover, moving down ofthe blank holder only needs to be performed by the time the pushing inby the second punch is completed.

However, when a low-strength steel sheet having high ductility is usedfor the steel sheet, the pushing in on the blank material S by the firstpunch 12 may be completed prior to the pushing in on the blank materialS by the second punch 13. In short, since the production method of thepresent embodiment uses a divided punch, it is possible to press formthe door inner panel into shapes of various degrees of formingdifficulty.

The production method of the present embodiment has described a case inwhich a door inner panel is produced by a hot stamping apparatusincluding a first and second punches. However, the number of punches isnot limited to two. The second punch may be divided into multiplepunches. In short, a hot stamping apparatus including three or morepunches may be used. In this case, multiple step portions are providedon the vertical wall portion of the door inner panel

In the production method described above, the steel sheet may be atailored blank. Hereinafter, an example of the door inner panel which isproduced from a tailored blank will be described.

Fifth Embodiment

FIG. 12 is a perspective view of a door inner panel of a fifthembodiment. FIG. 13A and FIG. 13B are perspective views each showing astarting material of the door inner panel shown in FIG. 12. Of thesefigures, FIG. 13A shows a state being punched an opening portion. FIG.13B shows a state after being punched an opening portion, andimmediately before subjected to hot stamping.

Compared with the door inner panels of the first to fourth embodimentsdescribed above, the door inner panel of the fifth embodiment shown inFIG. 12 is similar in that it is formed by hot stamping as describedabove, but is different in that it uses a tailored blank as the startingmaterial. The tailored blank is broadly classified into a tailoredwelded blank (hereinafter referred to as “TWB”) and a tailored rolledblank (hereinafter also referred to as “TRB”). TWB is a blank in whichmultiple kinds of steel sheets having different sheet thicknesses,tensile strengths, etc. are integrated together by welding (for example,butt welding). On the other hand, TRB is a blank in which the sheetthickness is varied by changing the spacing of the rolling rolls in theproduction of the steel sheet. FIGS. 13A and 13B show an exemplary casein which the tailored blank is TRB.

Referring to FIG. 12, in the door inner panel 1 of the fifth embodiment,a step portion 5 is provided in all of four adjacent vertical wallportions 4 (three pairs of vertical wall portions 4) as in the doorinner panel 1 of the second embodiment shown in FIG. 5. The openingportion 3 is provided at one place so as to leave a peripheral edgeportion of the top panel portion 2. In this door inner panel 1, thesheet thickness of a region of an edge portion (beltline portion 21) onthe vehicle upper side including a border (beltline BL) on the vehicleupper side of the top panel portion 2 is larger than that of a regionadjacent to this region. In other words, the sheet thickness of the doorinner panel 1 is not constant, and the sheet thickness of the beltlineportion 21 is increased. As a result, the strength of the beltlineportion 21 is increased so that the collision characteristic of the doorinner panel is improved. In addition, when there is a region other thanthe beltline portion 21, in which high strength is not required, it isexpected to reduce the weight of the door inner panel 1 by decreasingthe sheet thickness of that region.

The door inner panel 1 of the fifth embodiment is produced by using astarting material (TRB) shown in FIGS. 13A and 13B. Specifically, first,a TRB 30 having a contour shape corresponding to the contour shape ofthe door inner panel 1 is prepared as shown in FIG. 13A. In this TRB 30,the sheet thickness of a region corresponding to the beltline portion 21of the door inner panel 1 is larger than those of other regions. Next,the TRB 30 is formed with an opening portion 31 corresponding to theopening portion 3 of the top panel portion 2 of the door inner panel 1.This opening portion 31 is formed, for example, by punching. Bysubjecting the TRB 30 having such opening portion 31 to the hot stampingas described above, it is possible to form the door inner panel 1 asshown in FIG. 12.

In the fifth embodiment, a TWB may be used, as the starting material, inplace of the TRB.

Sixth Embodiment

FIG. 14 is a perspective view of a door inner panel of a sixthembodiment. The door inner panel 1 of the sixth embodiment shown in FIG.14 is a modification of the door inner panel of the fifth embodimentshown in FIG. 12 modified according to the third embodiment.

Referring to FIG. 14, in the door inner panel 1 of the sixth embodiment,as with the door inner panel 1 of the third embodiment shown in FIG. 6,a recessed portion 7 is provided in the beltline portion 21 of the toppanel portion 2 along the beltline portion 21 as in the door inner panel1 of the third embodiment shown in FIG. 6. Since the recessed portion 7is provided in the beltline portion 21 having a large sheet thickness,the strength of the beltline portion 21 is further increased. Thus, thebeltline portion 21 also plays a role of a beltline reinforcement. Thebeltline portion 21 may be provided either with a protruded portion inplace of the recessed portion 7, or with both the recessed portion 7 andthe protruded portion.

The door inner panel 1 of the sixth embodiment is produced by using thestarting material shown in FIGS. 13A and 13B as in the fifth embodiment.The recessed portion 7 is formed by hot stamping.

Seventh Embodiment

FIG. 15 is a perspective view of a door inner panel of a seventhembodiment. FIGS. 16A and 16B are perspective views each showing thestarting material of the door inner panel shown in FIG. 15. Of thesefigures, FIG. 16A shows a state before being punched an opening portion.FIG. 16B shows a state after being punched the opening portion, andimmediately before being subjected to hot stamping. The door inner panel1 of the seventh embodiment shown in FIG. 15 is a modification of thedoor inner panel 1 of the fifth embodiment shown in FIG. 12 modifiedaccording to the fourth embodiment.

Referring to FIG. 15, in the door inner panel 1 of the seventhembodiment, a plurality of opening portions 3A and 3B are provided inthe top panel portion 2 as in the door inner panel 1 of the fourthembodiment. FIG. 15 shows an exemplary case in which two openingportions 3A and 3B are provided. The top panel portion 2 has a boundaryportion 22 between the opening portions 3A and 3B. The boundary portion22 extends in a fore-and-aft direction substantially parallel to thebeltline portion 21. In this door inner panel 1, the sheet thickness ofa region of the beltline portion 21 of the top panel portion 2 is largerthan that of a region adjacent to this region. Further, the sheetthickness of a region of the boundary portion 22 of the top panelportion 2 is larger than that of a region adjacent to this region. Inother words, the sheet thickness of the door inner panel 1 is notconstant, but the sheet thicknesses of the beltline portion 21 and theboundary portion 22 are increased. As a result, the strength of thebeltline portion 21 and the boundary portion 22 is increased, and thecollision characteristic of the door inner panel is improved. Moreover,when there is a region other than the boundary portion 22, in which highstrength is not required, it is expected to reduce the weight of thedoor inner panel 1 by reducing the sheet thickness of that region.

The door inner panel 1 of the seventh embodiment is produced by usingthe starting material shown in FIGS. 16A and 16B. Specifically, first, aTRB 30 having a contour shape corresponding to the contour shape of thedoor inner panel 1 is prepared as shown in FIG. 16A. In this TRB 30, thesheet thickness of regions respectively corresponding to the beltlineportion 21 and the boundary portion 22 of the door inner panel 1 arelarger than those of other regions. Next, the TRB 30 is formed withopening portions 31A and 31B corresponding to the opening portions 3Aand 3B of the top panel portion 2 of the door inner panel 1. By applyingthe hot stamping described above to the TRB 30 having such openingportions 31A and 31B, it is possible to form the door inner panel 1shown in FIG. 15.

The door inner panel 1 of the seventh embodiment may also be varied suchthat the sheet thickness of either one of the beltline portion 21 andthe boundary portion 22 is increased.

Eighth Embodiment

FIG. 17 is a perspective view of a door inner panel of an eighthembodiment. The door inner panel 1 of the eighth embodiment shown inFIG. 17 is a modification of the door inner panel 1 of the seventhembodiment shown in FIG. 15 modified according to the fourth embodiment.

Referring to FIG. 17, in the door inner panel 1 of the eighthembodiment, a recessed portion 8 is provided in the boundary portion 22of the top panel portion 2, as in the door inner panel 1 of the fourthembodiment, along the boundary portion 22. Since the recessed portion 8is provided in the boundary portion 22 having a large sheet thickness,the strength of the boundary portion 21 is further increased. As aresult, the boundary portion 22 also plays a role of a door impact beam.The boundary portion 22 may be provided with a protruded portion inplace of the recessed portion 8, or both of the recessed portion 8 andthe protruded portion.

The door inner panel 1 of the eighth embodiment is produced by using thestarting material shown in FIGS. 16A and 16B as in the seventhembodiment. The recessed portion 8 is formed by hot stamping.

Ninth Embodiment

FIG. 18 is a perspective view of a door inner panel of a ninthembodiment. FIGS. 19A and 19B are each a perspective view showing thestarting material of the door inner panel shown in FIG. 18. Of thesefigures, FIG. 19A shows a state before being punched an opening portion.FIG. 19B shows a state after being punched the opening portion andimmediately before being subjected to hot stamping.

Referring to FIG. 18, in the door inner panel 1 of the ninth embodiment,the sheet thickness of a region of a lower-side vertical wall portion 4including a lower-side border 2B of the top panel portion 2 is largerthan that of a region adjacent to that region. In other words, the sheetthickness of the door inner panel 1 is not constant, but the sheetthickness of the lower-side vertical wall portion 4 is increased. As aresult, strength of the lower-side vertical wall portion 4 is increased,and the collision characteristic of the door inner panel is improved. Inaddition, when there is a region other than the lower-side vertical wallportion 4, in which high strength is not required, it is also expectedto reduce the weight of the door inner panel 1 by reducing the sheetthickness of that region. FIG. 18 shows an exemplary case in which thesheet thickness is increased in a lower-side step portion 5 and alower-side flange portion 6, in addition to the lower-side vertical wallportions 4A and 4B.

The door inner panel 1 of a ninth embodiment is produced by using thestarting material shown in FIGS. 19A and 19B. Specifically, first, a TRB30 having a contour shape corresponding to the contour shape of the doorinner panel 1 is prepared. In this TRB 30, the sheet thickness of aregion corresponding to a lower-side vertical wall portion 4 of the doorinner panel 1 is larger than those of other regions. Next, the TRB 30 isformed with an opening portion 31 corresponding to the opening portion 3of the top panel portion 2 of the door inner panel 1. By applying thehot stamping described above to the TRB 30 having such opening portion31, it is possible to form the door inner panel 1 shown in FIG. 18.

Note that the door inner panel 1 of the fifth to eighth embodiment mayalso be varied such that the sheet thickness of lower-side vertical wallportion 4 is increased according to the ninth embodiment.

FIG. 20 is a perspective view of a door inner panel having an openingportion 3 having a shape different from that of FIG. 18. FIG. 21, whichis a perspective view showing the starting material of the door innerpanel shown in FIG. 20, shows a state after being punched the openingportion and immediately before being subjected to hot stamping.

In the door inner panel 1 shown in FIG. 20, an opening portion 3 extendsover one border of a peripheral edge portion of a top panel portion 2.In other words, the opening portion 3 is provided so that a beltline BLis interrupted. In this case, as shown in FIG. 21, an opening portion 31corresponding to the opening portion 3 of the top panel portion 2 of thedoor inner panel 1 is formed in the TRB 30 before the punchingprocessing shown in FIG. 19A. By applying the hot stamping describedabove to the TRB 30 having such opening portion 31, it is possible toform the door inner panel 1 shown in FIG. 20.

Tenth Embodiment

FIG. 22 is a perspective view of a door inner panel of a tenthembodiment. FIGS. 23A and 23B are each a perspective view showing astarting material of the door inner panel shown in FIG. 22. Of thesefigures, FIG. 23A shows a state before being punched an opening portion.FIG. 23B shows a state after being punched the opening portion andimmediately before being subjected to hot stamping.

Referring to FIG. 22, in a door inner panel 1 of a tenth embodiment, thesheet thickness of a region of a front-side vertical wall portion 4(vertical wall portion 4A) including a front-side border 2A of the toppanel portion 2 is larger than that of a region adjacent to the region.In other words, the sheet thickness of the door inner panel 1 is notconstant, but the sheet thickness of the front-side vertical wallportion 4 is increased. As a result, the strength of the front-sidevertical wall portion 4 to which a hinge is attached is increased. Inaddition, when there is a region other than the front-side vertical wallportion 4, in which high strength is not required, it is expected toreduce the weight of the door inner panel 1 by decreasing the sheetthickness of that region. FIG. 22 shows an exemplary case in which inaddition to the front-side vertical wall portions 4A and 4B, thethicknesses of the front-side step portion 5 and the front-side flangeportion 6 are also increased.

The door inner panel 1 of a tenth embodiment is produced using thestarting material shown in FIGS. 23A and 23B. Specifically, first, asshown in FIG. 23A, a TRB 30 having a contour shape corresponding to thecontour shape of the door inner panel 1 is prepared. In this TRB 30, thesheet thickness of a region corresponding to the front-side verticalwall portion 4 of the door inner panel 1 is larger than those of otherregions. Next, the TRB 30 is formed with an opening portion 31corresponding to the opening portion 3 of the top panel portion 2 of thedoor inner panel 1. By applying the above described hot stamping to theTRB 30 having such opening portion 31, it is possible to form the doorinner panel 1 shown in FIG. 22.

Note that using a TWB as the starting material makes it possible to varythe door inner panel 1 of the fifth to ninth embodiments according tothe present embodiment such that the sheet thickness of the front-sidevertical wall portion 4 is increased. This is because the TWB has alarger degree of freedom for combining steel sheets.

In the above description, a case in which the material of the door innerpanel is a steel sheet is described. However, the material of the doorinner panel is not limited to a steel sheet, but any metal sheet may beused. The metal sheet may be made of, for example, aluminum, an aluminumalloy, a multi-layer steel sheet, titanium, magnesium, or the like.Further, in the above description, a case in which the panel-shapedformed product is a door inner panel is described. However, thepanel-shaped formed product is not limited to the door inner panel. Thepanel-shaped formed product can be applied to a product requiringexcellent impact characteristics. Such products include, for example,vehicles, construction machines, aircraft, etc. in addition toautomobiles.

EXAMPLES

Analysis of each press working was conducted supposing press working byuse of the hot stamping apparatus 10 shown in FIGS. 9A and 11A, and thehot stamping apparatus 200 shown in FIG. 10. From analysis results,sheet thickness reduction rate and curvature distribution of the doorinner panel obtained by each press working were evaluated. Here, asInventive Example 1 of the present invention, press working by use ofthe hot stamping apparatus 10 shown in FIG. 9A was supposed. AsInventive Example 2 of the present invention, press working by use ofthe hot stamping apparatus 10 shown in FIG. 11A was supposed. AsComparative Example, press working by use of the hot stamping apparatus200 shown in FIG. 10 was supposed.

[Analysis Condition]

The blank material was a steel sheet containing, by mass %, C: 0.21%,Si: 0.25%, Mn: 1.2%, and B: 0.0014% with the balance being Fe andimpurities. The material characteristics after quenching were supposedto be a Vickers hardness of 448, a yield strength of 448 MPa, a tensilestrength of 1501 MPa, and a breaking-off elongation of 6.4%. The heatingtemperature of the blank material was set to 750° C. In consideration ofstrain rate dependency of mechanical properties, the moving speed of thefirst and second punches relative to the die was set to equivalent of 40m/s. The friction coefficient of the blank material against punch, thedie, and the blank holder was supposed to be 0.4. Analysis was conductedby using heat-forming coupled analysis by a general purpose FEM (FiniteElement Method) software (Trade name: LS-DYNA, supplied by LIVERMORESOFTWARE TECHNOLOGY Corporation) was used.

FIG. 24 shows dimensions of the die used in the analysis of the presentexamples. The unit of dimensions in FIG. 24 is mm. A depth d1 between areference surface 16C of the die and a step surface 16B was supposed tobe 45 mm. And a depth d2 between the reference surface 16C of the dieand a die bottom surface 16A was supposed to be 120 mm.

In Inventive Example 1 of the present invention, arrangement was madesuch that the first punch and the second punch came into contact withthe blank material at the same time. In other words, in InventiveExample 1 of the present invention, when pushing in by the second punch13 was completed as shown in FIG. 9B, the front end surface 12A of thefirst punch 12 was located at the same height as the front end surface13A of the second punch 13. In Inventive Example 2 of the presentinvention, the front end surface of the first punch was located lowerthan the front end surface of the second punch. In other words, inInventive Example 2 of the present invention, when pushing in by thesecond punch 13 was completed as shown in FIG. 11B, the front endsurface 12A of the first punch 12 was set to be located 40 mm lower thanthe front end surface 13A of the second punch 13. In ComparativeExample, a punch which was integrated without being separated was used.In other words, the step portion and the top panel portion of the doorinner panel were formed by a single punch as shown in FIG. 10. Moreover,in both Inventive Examples 1 and 2 of the present invention, it wasarranged such that the blank material was held by the die and the blankholder before the first and second punches pushed in the blank material.

[Evaluation Method]

Sheet thickness reduction rates of the door inner panel and curvaturedistribution of the surface thereof, which were obtained by the analysisof each press forming, were investigated. The sheet thickness reductionrate was calculated using the following Formula (1).(Sheet thickness reduction rate [%])=((sheet thickness before pressforming)−(sheet thickness after press forming))/(sheet thickness beforepress forming)×100   (1)

The curvature was calculated using the following Formula (2).(Curvature[1/m])=(1/(radius of curvature))  (2)

Here, a radius of curvature of the surface of the door inner panel wascalculated, at each position, in each of a plurality of cross sectionsperpendicular to the surface of the door inner panel, and a minimumvalue thereof was adopted for the radius of curvature of Formula (2).The curvature was determined to be a positive value when the material ofeach position was deformed in such a way to be convex to the backsurface side, and a negative value when it was deformed in such a way tobe convex to the font surface side. Note that the curvature wasevaluated when the first punch reached a position at a distance of 1 mmfrom its bottom dead center. When the sheet thickness reduction rate isnot less than 20.0%, it was judged that a crack occurred in the doorinner panel. When the absolute value of the curvature was not less than0.01, it was judged that a wrinkle occurred in the door inner panel.

[Analysis Result]

FIGS. 25A and 25B show analysis results of Inventive Example 1 of thepresent invention. FIG. 25A shows the sheet thickness reduction rate ofthe door inner panel of Inventive Example 1 of the present invention.FIG. 25B shows the curvature distribution of the door inner panel ofInventive Example 1 of the present invention. Referring to FIG. 25A, amaximum value of the sheet thickness reduction rate was 16.5% inInventive Example 1 of the present invention. Therefore, it can be saidthat no crack occurred in the door inner panel. Referring to FIG. 25B,there was no portion observed in which the absolute value of thecurvature was not less than 0.01 in the step portion of the door innerpanel in Inventive Example 1 of the present invention. Therefore, it canbe said that no wrinkle occurred.

FIGS. 26A and 26B show analysis results of Inventive Example 2 of thepresent invention. FIG. 26A shows the sheet thickness reduction rate ofthe door inner panel of Inventive Example 2 of the present invention.FIG. 26B shows the curvature distribution of Inventive Example 2 of thepresent invention example 2. Referring to FIG. 26A, in Inventive Example2 of the present invention, the maximum value of the sheet thicknessreduction rate was 15.8%. Therefore, it can be said that no crackoccurred in the door inner panel. Referring to FIG. 26B, in InventiveExample 2 of the present invention, there was no portion observed inwhich the absolute value of the curvature was not less than 0.01 in thestep portion of the door inner panel. Therefore, it can be said that nowrinkle occurred.

FIGS. 27A and 27B shows analysis results of Comparative Example. FIG.27A shows the sheet thickness reduction rate of the door inner panel ofComparative Example. FIG. 27B shows the curvature distribution of thedoor inner panel of Comparative Example. Referring to FIG. 27A, inComparative Example, the maximum value of the sheet thickness reductionrate was 14.0%. Therefore, it can be said that no crack occurred in thedoor inner panel. Referring to FIG. 27B, in Comparative Example, therewas observed a portion in which the absolute value of the curvature wasnot less than 0.01 in a region indicated by X in FIG. 27B in the stepportion of the door inner panel. Therefore, it can be said that wrinklesoccurred.

So far the embodiments of the present invention have been described.However, the embodiments described above are merely examples forcarrying out the present invention. Accordingly, the present inventionwill not be limited to the above-described embodiments, but theabove-described embodiments can be appropriately altered and carried outwithin a range not departing from the spirit thereof.

REFERENCE SIGNS LIST

-   1 Door inner panel (panel-shaped formed product)-   2 Top panel portion-   21 Beltline portion-   22 Boundary portion-   3, 3A, 3B, 31, 31A, 31B Opening portion-   4, 4A, 4B Vertical wall portion-   5 Step portion-   6 Flange portion-   7, 8 Recessed portion-   10 Hot stamping apparatus-   12 First punch-   13 Second punch-   14 Blank holder-   15 Die-   30 TRB-   BL Beltline-   S Blank material

The invention claimed is:
 1. A method for producing a panel-shapedformed product, wherein the panel-shaped formed product, wherein thepanel shaped product includes a polygonal top panel portion, verticalwall portions, wherein each of the vertical wall portions extends fromeach adjacent border of the polygonal top panel portion, and a stepportion provided on each of the vertical wall portions, the productionmethod comprising: a preparation step of preparing a blank material madeof a steel sheet, the blank material having an opening portion at aposition corresponding to the top panel portion, a heating step ofheating the blank material, and a press forming step of applying pressworking by hot stamping forming to the heated blank material to form theblank material into the panel-shaped formed product having a quenchedmartensitic structure and a tensile strength of not less than 1200 MPa,wherein the press forming step uses a press working apparatus thatincludes: a die having a die engraved portion in which the shape of thepanel-shaped formed product is formed, a first punch having a front endsurface which faces the die and in which the shape of the top panelportion is shaped, a second punch which is adjacent to outside of thefirst punch and has a front end surface in which the shape of the stepportion is shaped, and a blank holder existing adjacent to at least apart of the outside of the second punch and facing the die, wherein theblank material is positioned between the die and the blank holder, theblank material is positioned between the die and the first punch, andthe blank material is positioned between the die and the second punch,and the blank holder, the first punch, and the second punch arerelatively moved with respect to the die to push in the first punch andthe second punch on the blank material, wherein pushing in on the blankmaterial by the second punch is completed prior to completion of pushingin on the blank material by the first punch thereby forming the blankmaterial into the panel-shaped formed product.
 2. The method forproducing the panel-shaped formed product according to claim 1, whereinin the press forming step, when the pushing in on the blank material bythe second punch is completed or after the pushing in is completed,pushing in on the blank material by the first punch is started.
 3. Themethod for producing a panel-shaped formed product according to claim 1,wherein in the press forming step, before pushing in on the blankmaterial by the second punch is completed, pushing in on the blankmaterial by the first punch is started.
 4. The method for producing apanel-shaped formed product according to claim 1, wherein the dieengraved portion of the die used in the press forming step satisfiesconditions: d2≥40 mm, and d1/d2<0.8, where d1 is a depth from areference surface facing the blank holder to a step surface facing thesecond punch, and d2 is a depth from the reference surface to a diebottom surface facing the first punch.
 5. The method for producing apanel-shaped formed product according to claim 1, wherein the steelsheet is a tailored blank.